brown algae

Brown algae are Organisms belonging to the phylum Phaeophyta of the kingdom Protista.
Of the approximately 1,500 species of brown algae known, almost all are
marine. They include many seaweeds, including the familiar wracks and bladder
wrack, and the kelps, including the largest of all brown algae, Macrocystis
– a genus of kelp that grows to more than 100m (320ft) at up to 0.5
(18in) per day. Sargassum forms vast floating masses in the Sargasso
Sea in the mid-Atlantic, with their own distinctive communities of animals
and microorganisms.

Brown algae are very diverse in form and size, ranging from less than 1
mm long to some species that are among the largest photosynthetic organisms
on Earth. The largest kelps may grow to more than 60 meters in length, forming
dense underwater forests in colder waters. Many microscopic brown algae
grow as epiphytes on underwater vegetation, forming networks of branched
filaments, or broad encrustations. All species are multicellular and do
not form colonies. Their life cycles are complex, involving alternation
of generations. In general, they are not free-floating organisms, but are
attached to rock, coral, or other firm surfaces. A species of Sargassum is exceptional in being pelagic (open-sea dwelling), accumulating in large
quantities in the Sargasso Sea near the West Indies and staying afloat by
means of gas-filled bladders.

The main food reserve of brown algae is the polysaccharide laminarin. The cell wall consists of two layers: an inner
one of cellulose and an outer one of mucilaginous
pectic material. Asexual reproduction occurs by fragmentation of thallus or by mostly laterally biflagellate zoospores.
Sexual reproduction is isogamous, gametes motile, or anisogamous.